Microsatellite instability occurs due to a defect in mismatch repair. This is usually a result of inactivation of MSH2, MLH1 or MSH6 due to a mutation or to reduced expression associated with promoter methylation. Analysis of microsatellite instability (MSI) was carried out according to the guidelines set down by "The International Workshop on Microsatellite Instability and RER Phenotypes in Cancer Detection and Familial Predisposition" workshop as published by Boland et al. As recommended in the workshop all samples were screened using the markers BAT25, BAT26, D5S346, D2S123 and D17S250. Samples were characterised as high-frequency MSI (MSI-H) if two or more markers showed instability and low-frequency MSI (MSI-L) if one marker of the five showed signs of instability. Microsatellite stable (MSS) samples and low-frequency MSI can only be distinguished by running further markers. The classification for each sample is shown, together with the data for each of the five markers (microsatellite instability is shown as a 1 with stable calls as 0 and failed markers as 'f').
Boland, C.et al., A National Cancer Institute Workshop on Microsatellite Instability for Cancer Detection and Familial Predisposition: Development of International Criteria for the Determination of Microsatellite Instability in Colorectal Cancer. Cancer Research (1998); 58, 5248-5257.